The writing of optical waveguides (i.e., light-guiding or light-managing structures) embedded within glass is accomplished by focusing high energy pulses within the glass and relatively translating the pulses with respect to the glass. The high-energy pulses have wavelengths beyond the absorption edge (optical density of unity) of the glass, but are focused to power densities within the glass at which the glass undergoes a change in refractive index. Thus, the pulses can penetrate the glass without dispersing their energy or damaging the glass, but can be focused to threshold power densities at which localized portions of the glass undergoes a transformation that changes the local refractive index of the glass.
The pulses are delivered in beams that are focused to near the diffraction limit to concentrate pulse energies within limited spot sizes. The resulting refractive index changes are centered at the spot focus of the beams. The spot focus can be relatively translated to produce modified-index tracks through bulk glasses. Index increases in the range of 1×10−3 to 5×10−3 have been found sufficient to support waveguiding properties along the tracks, which function as the cores of waveguides.
Two writing regimes have emerged for writing within different types of glass materials. One, which relies on amplified femtosecond pulse sources, operates at pulse energies in the microjoule (μJ) range and at repetition rates in the kilohertz (kHz) range. Another, which relies on femtosecond laser oscillators, operates at pulse energies in the nanojoule (nJ) range and at repetition rates in the megahertz (MHz) range. In both regimes, non-linear absorption mechanisms are believed responsible for producing localized increases in refractive index.
Of the two regimes, cores can be written most effectively in silica-based glasses using amplified femtosecond pulse sources. Femtosecond laser oscillators can write waveguides more effectively in materials such as borosilicate, sulfide, and lead glasses. However, in both instances, the desired localized increases in refractive index tend to be self-limiting in the range of a multiple of 10−3.
The spot focuses of the femtosecond lasers generally produce small diameter cores of around 2 microns (μm) to 3 microns (μm). Given the index change on the order of 10−3, the waveguiding properties of the small diameter waveguides are relatively weak. Where, as here, both the index difference and the waveguide diameter are comparatively small, a substantial portion of the signal is carried in the surrounding medium (e.g., a cladding) of the waveguide. Signal losses are larger for signals that significantly encroach upon the surrounding medium. Bending losses associated with such weak waveguiding properties limit the minimum radius of curvature through which the waveguides can be bent during use.
Co-assigned U.S. patent application Ser. No. 09/997,751 entitled Manipulating the Size of Waveguides Written into Substrates using Femtosecond Pulses, which is hereby incorporated by reference, describes additional motions for enlarging the core areas of waveguides to improve light-guiding efficiency. However, other consequences of enlarged diameters include different requirements for coupling to the waveguides and transmission of additional modes.